Regardless of whether or not the speakers on the mini actually are stereo, the Kindle Fire HD's speakers are remarkably good for a tablet of that size, and the fact they are on the left and right when it's held in landscape mode means you get genuine stereo.
I have one, I watch movies on it, and I can assure you that the iPad mini will not sound as good, because no matter how amazing those speakers are, you won't perceive stereo. So, regardless of whether or not the iPad mini has two channel sound, it is stereo only in the most technical of ways. Speakers an inch apart aren't likely going to give any perceived separation of left and right, even in portrait mode, and certainly won't in landscape, which is how almost all movies are watched.
The iPad mini wins on build quality, weight, and to some extent the app store (for most people, even the Amazon app store is not all that limited). The Kindle Fire HD wins on price, sound, resolution, HDMI output, and ease of charging ports (micro USB trumps Lightning).